The present invention relates to steam turbine plants and more particularly to electric power plants operated by steam turbines for which the steam supply is provided by a nuclear boiling water reactor.
In a boiling water nuclear reactor, the nuclear fuel is structured with a suitable geometry to provide for a sustained chain nuclear reaction as the coolant water passes through the fuel arrangement. Conventionally, the nuclear fuel is housed in elongated metallic tubes which are in turn assembled and supported in parallel arrays or bundles. The reactor core is formed from an assembly of the fuel bundles, and it is housed in a large pressure vessel with provision for coolant flow along all of the fuel elements. Neutron absorbing control rods are supported within the core for movement relative to the fuel elements.
The design of the core and other reactor parameters determine the reactor power rating. Mechanical, nuclear, hydraulic and other details of the reactor design are the result of development programs aimed at achieving efficient performance for the plant owner.
Since water density is a large determinant of the rate of generation of slow neutrons which are required for the controlled propagation of the chain nuclear reaction, the power operating level of the reactor is partly determined by the accumulation of steam voids in the core volume. Increased coolant flow causes faster fuel rod cooling with reduced boiling and, accordingly, reduced void accumulation and higher reactor power. Decreased coolant flow has the opposite effects. Typically, coolant flow control can be used to control the boiling water reactor power level within a range of about 20% or 25% with a preset control rod placement.
The reactor generated steam is normally directed through separators and dryers within the pressure vessel, and the dry saturated steam is directly channeled at a pressure such as 1000 psi and a temperature such as 545.degree.F to the utilization equipment, i.e., the turbine generator unit(s) of the electric power plant. Separated water is combined in the pressure vessel with external and internal recirculation flows and with return and makeup feedwater flow.
Since the boiling water reactor plant is the direct cycle type and since outlet steam pressure and reactor vessel pressure affect the void accumulation in the reactor core, it is desirable to operate the turbine inlet valves to determine the turbine and generator load level subject to pressure regulating demands of the reactor. With reactor pressure maintenance within a relatively narrow pressure band such as about 30 psi, reactor power level is controlled by coolant flow control within a limited range or by control rod movement if a different power range is required to meet load demand on the turbine generator unit(s).
In general, the steam turbine energization level is determined by the flow of the turbine inlet steam which in turn is determined by the steam conditions at the outlet of the steam source and by steam inlet valve positioning. The turbine drive power supplied for the plant generators is desirably controlled to satisfy electrical load demand and frequency participation demand placed on the electric power plant by the plant operator or by an economic dispatch computer or by other means.
At substantially constant temperature throttle steam, turbine power is porportional to turbine steam flow, and if the throttle pressure is also substantially constant, the steam flow is proportional to impulse chamber steam pressure or the ratio of the impulse chamber steam pressure to the throttle steam pressure. As already indicated, positioning of the inlet steam valving must provide for reactor vessel pressure regulation as well as turbine energization level control. When the boiling water reactor power level corresponds to the plant load demand, the turbine inlet valves are positioned to produce both the desired reactor vessel pressure and the turbine steam flow required for satisfying plant electrical load demand.
A steam bypass system is also usually provided to direct steam flow from the reactor outlet to the plant condenser under certain conditions. Steam bypass in effect provides an interface between the boiling water reactor and the steam turbine during reactor startup and shutdown and during other periods such as during load rejection. In these cases, steam supplied by the reactor but not needed by the turbine is channeled to the condenser under control imposed on the bypass system by the throttle pressure control system.
To control a boiling water reactor-steam turbine plant, it has been customary to use the turbine follow mode of operation. After plant startup, corrective changes are made in the reactor power level by automatic or manual reactor coolant flow control or by manual or possibly automatic control rod operation in order to satisfy plant load demand. Turbine throttle pressure is sensed and the turbine inlet steam valves are operated in the follow mode to control the throttle and reactor vessel pressures and enable turbine steam flow changes to be made to correct the turbine load as the reactor power level is being corrected. To speed up the control, particularly when step changes are made in load demand, the setpoint of the turbine pressure control may be temporarily adjusted in response to the load error.
However, when the turbine and reactor are in automatic control and the recirculation system is either at its low or high limit, a change of load demand or load reference and the resultant movement of the valve position will not result in the desired sustained change of load level since the reactor output is limited. Such a condition causes an excessive deviation of throttle pressure and tends toward unstable reactor operation until the control rods are repositioned.
In the Podolsky U.S. Pat. No. 3,630,839, there is described another kind of control system for boiling water reactor-steam turbine plants. The Podolsky control is a coordinated control system which applies load demand directly to the turbine inlet valve controls as well as the reactor controls to produce better turbine and plant performance within throttle pressure constraints.
In the typical boiling water reactor-steam turbine application, the part of the control system directed to turbine valve control is principally mechanical and hydraulic in character with some electrical circuitry such as that involved in the throttle pressure sensing function. Examples of principally hydraulic turbine inlet valve feedback controls in nonnuclear applications are set forth in U.S. patents to Bryant 2,552,401 and Marlsand 1,777,470. A principally mechanical turbine inlet valve feedback control is shown in U.S. patent to Eggenberger 3,027,137 in a nonnuclear application. Electrohydraulic analog feedback type turbine inlet valve controls have been employed in nonnuclear turbine applications to achieve operational improvements, and examples of such controls are presented in U.S. patents to Bryant 2,262,560, Herwald 2,512,154, Eggenberger 3,097,488, 3,097,489, 3,098,176 and Callan 3,097,490. Further details on conventional electrohydraulic control in nonnuclear applications are presented in a paper entitled "Electrohydraulic Control For Improved Availability And Operation Of Large Steam Turbines" and presented by M. Birnbaum and E. G. Noyes to the ASME-IEEE National Power Conference at Albany, New York during September 19-23, 1965.
In U.S. Patent No. 3,630,839 to Podolsky and U.S. Patent 3,671,390 to Hogle, there is also disclosed a control system for boiling water reactor-steam turbine plants in which electrohydraulic controls are employed to produce better plant operation than that produced by the typical mechanical-hydraulic controls. In the allowed U.S. Patent Application Serial No. 184,157 to Giras and Podolsky, there is disclosed a digital electrohydraulic turbine control system which employs feedforward control principles.
In order to attain the ultimate in flexibility of a digital system and the safety of operation of an analog system in the event of contingencies, it is desirable that such a control system for a boiling water reactor steam turbine plant employ both such systems to cooperate efficiently in the various operating modes, and to transfer from one to the other effectively in the event of certain contingencies of operation.